Apparatus for machining through varying-frequency constant-duration pulse-controlled electric discharges

ABSTRACT

Apparatus used for electric discharge machining, which senses the commencement of each discharge across the gap between the electrode and workpiece and cuts off each discharge after a predetermined time interval.

United States Patent Pfau Oct. 28, 1975 1 APPARATUS FOR MACHININGTHROUGH [52] U.S. Cl 219/69 C; 219/69 P VARYING-FREQUENCY [51] Int. Cl.B23P l/08 CONSTANT DURATION [58] Field of Search 219/69 C, 69 PPULSE-CONTROLLED ELECTRIC DISCHARGES [56] References Cited [75]Inventor: Jean Pfau, Geneva, Switzerland UNITED STATES PATENTS 3,259,7957/1966 Schierholt 219/69 C [73] Assigneez Ateliers Des Charmilles, S.A.,

Geneva, swiilerland Primary Examiner-Bruce A. Reynolds 22 Filed; Man 3,v1970 Attorney, Agent, or Firml-lauke, Patalidis & Dumont [21] Appl.No.: 18,803 [57] ABSTRACT Related U.S. Application Data Apparatus usedfor electric discharge machining, [62] Division f Sen No 423,237, Jam 471965 which senses the commencement of each discharge across the gapbetween the electrode and workpiece [30] Foreign Application priorityData and cuts off each discharge after a predetermined time interval.

Feb. 25, 1964 Switzerland 2295/64 24 Claims, 12 Drawing Figures 7' F2 f-f n M 0 fl 3 15 4 I i W: I z :5 I 1 I l i l 1 1PI I: ill 1 Y1 YE I I il I I I 1 II in 1 I l {1 1 1: vi 1 i111 111:: Y 111, 1 II I I: I: l 1.:l

: 4 1': 1: 1: 61 6. 1,1 1,: 1,: P4, 11,, 1 12mm,

US. Patent Oct. 28, 1975 Sheet ,2 of 3 3,916,138

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FIG. lb

US. Patent Oct. 28, 1975 Sheet30f3 3,916,138

APPARATUS FOR MACHINING THROUGH VARYING-FREQUENCY CONSTANT-DURATIONPULSE-CONTROLLED ELECTRIC DISCHARGES This application is a division ofmy copending application Ser. No. 423,237, filed Jan. 4, 1965.

Various methods for machining through intermittent electric dischargesbetween a part of conductive material to be machined and an electrodehave already been proposed.

The simplest apparatus for the execution of such methods are thosemachining apparatus of the relaxation type, wherein a condenserconnected in parallel between the part to be machined and the electrodeis charged from source and then unloaded into the machining gap eachtime its voltage reaches a value sufficient for the jumping of a sparkbetween the electrode and said part.

Apparatus of such a type have such drawbacks that in spite of theirgreat simplicity, they are used less and less and the operator alwaysprefer more improved apparatus. One of the chief drawbacks of theapparatus referred to resides in their low machining speed which islimited for each predetermined surface condition by the production ofshort circuits as soon as it is attempted to increase the frequency byincreasing the current charging the condenser.

To remove this drawback, it has already been proposed to control theintermittent discharges by applying periodically across the electrodeand the part to be machined a difference in potential adapted to make aspark jump and by opening periodically the discharge circuit.

According to this method, the application of said difference inpotential is performed at a predetermined frequency and during apredetermined duration so that the current pulses to be obtained shouldapparently all be similar. Now, experience shows that the intermittentdischarges are not equal as to grade and duration and this isasoi'ibable to the fact that the discharges are not obtainedinstantaneously as soon as the necessary voltage is applied between theelectrode and the part to be machined. There exists, as a matter offact, an undefined delay which varies from one discharge to the next.

The average value of this delay depends on numerous parameters, chieflyon the voltage applied to the machining gap, on the distance separatingthe electrode from the part or work to be machined and on the energy ofthe discharge. This delay plays only a negligible part when thefrequency of the machining pulses is reduced and, in principle, isreduced down to less than about 1 kilocycle. ln contradistinction, ifthemachining is performed at a high frequency, chiefly for instance at tensof kilocycles, the delay may become comparable to the duration of apulse.

This is illustrated in accompanying FIGS. la and lb.

FIG. la illustrates the voltage pulses applied between the part to bemachined and the electrode, and

FIG. 1b illustrates the current pulses which are discharged between theelectrode and the part. The voltage pulses have always the same durationand are alway spaced equally. Their periodicity T is equal to theinverse of the frequencyf; at the beginning of each pulse, the voltageis high, until commencement of the discharge at which time the voltagedrops down to the value of the discharge voltage between the part andthe electrode.

During the first pulse the delay reaches of the du ration of the pulseTon, while during the second pulse this delay is equal to 0. For thethird pulse, the delay is larger than the duration of the actual pulse,so that no discharge occurs. The fourth pulse is similar to the firstone. As apparent from inspection of FIG. lb, only the second currentpulse shows the maximum possible duration Ton, the first and fourthpulse being shorter, while the third pulse is cut out. It should benoted that the frequency of the current pulses is lower than thefrequency of the voltage pulses.

The actual duration of each machining pulse is reduced by the durationof the delay and since the latter varies irregularly for each pulse, thepulses do not shown a uniform duration. The discharges are constitutedby a mixture of pulses of varying lengths and consequently of a variableenergy as apparent from FIG. 1b. Since the amount of material removed ateach discharge varies as a function of the energy of the discharge, themachining is obtained by a succession of craters of which the size isnot constant. The grade of machining is obviously defined by the size ofthe larger craters.

For a machining of a predetermined grade, it will be readily understoodthat all the discharges, of which the energy is lower than the maximumpossible energy which may be carried by each discharge, correspond to aloss of efficiency. Furthermore, when the duration of the delay iscomparable to or larger than the duration of a pulse, certain voltagepulses are not accompanied by any discharge current as in the case ofthe third pulse illustrated in FIG. la. This results in a considerablereduction of the average machining current with reference to that whichmight be obtained if the duration of the delay were reduced to 0. Thisreduction of the average current leads obviously to a correspondingreduction of the speed of machining.

An object of the invention is to obtain a maximum average machiningcurrent and consequently a maximum speed for a predetermined grade ofthe surface.

Another object of the present is to provide a method for machiningthrough intermittent electric discharges jumping between a part ofconductive material to be machined and an electrode, according to whichmethod controlled pulses, the voltage of which is higher than thedischarge voltage, are formed between the part to be machined and theelectrode, and produce, after an undefined delay, a discharge betweensaid part and the electrode, and the duration of each pulse increases asa function of the duration of the delay of the corresponding discharge.

The present invention has another object in that apparatus for machininga part by electrical discharge includes a generator of intermittentdischarges, means for controlling the discharges, a circuit detectingthe commencement of each discharge and means controlled by the circuitand acting on the means controlling the discharges, so as to end eachdischarge at a suitable moment.

In the accompanying drawings given by way of example.

FIGS. 1a and lb are explanatory graphs as referred to hereinabove.

FIGS. 2 to 6 illustrate diagrammatically and by way of example severalembodiments of apparatus for executing the improved method disclosedhereinabove. More specifically, FIG. 2 is a diagrammatic showing of oneembodiment. FIGS. 3a to 3fare explanatory graphs illustrating theoperationof the embodiment of FIG. 2. FIG. 4 is a circuit diagramof anembodiment of the present invention. FIGS. and 6 show two furtherembodiments of the present invention.

Turning first to FIG. 2, the apparatus includes a supply of current Sadapted to supply the energy required for the discharges to be producedbetween an electrode E and the part to be machined P. The supply S isconnected with the electrode E through a current making and breakingelement 1 and with the part P through a resistance R Resistance Rdetects the passage of current during the discharge between'theelectrodeE and the part P as provided by the voltage drop appearing across theterminals of resistance R when the discharge current passes through it.The voltage across the terminals of the resistance R, is applied to aSchmitt trigger 2,'i.e.- a binary circuit similar to a flip-flopcircuit, which rocks into one position when the input signal is largerthan a predetermined value and which returns to its former position onlywhen the input signal has been lowered to a value substantially lowerthan that which produced its first rocking movement. There is thusalways obtained a very sudden output signal, even when the value of theinput signal varies slowly. The Schmitt trigger 2 controls in its turn amonostable multivibrator 3 through the agency of a differentiatingcircuit 14 supplying short pulses during the rocking of the Schmitttrigger. The output of the multivibrator 3 is connected in its turn withthe differentiating circuit which controls a second multivibrator 4which is monostable. The multivibrator 4 controls the operation of thecurrent breaking and making element 1, or switch element, which isadvantageously constituted by at least one semi-conductive element, theconduction of which is controlled.

TheSchmitt trigger 2 and the multivibrators 3 and 4 are fed by a supplyS The operation of the apparatus FIG. .2 will nowbe explained with,reference to FIGS. 30 to 3f.

FIG. 3a illustrates the voltage E between the electrode and the part asa function of time and during a short period of operation correspondingto four discharges. At the time tI the current breaking and mak ingelement 1 is closed, so that the voltage of the supply S is appliedacross the electrode E and the part P. The discharge is however notobtained instantaneously, but only after a delay lasting from II to t2.the discharge is produced between :2 and 13 during a predeterminedduration T, and it is interrupted at the moment :3 by the current makingand breaking element I the control of which will be describedhereinafter. The duration of the interruption designated by Toff is alsoa predetermined value.

At the time [4, the voltage is again applied across the electrode andthe part and the discharge is obtained immediately, the delay beingassumed how to be practically equal to O. The discharge is interruptedat the time t5.

It will be assumed that the following discharge is only establishedafter a comparatively long delay extending from t6 to :7, the delaybeing longer than the desired duration of a discharge. The dischargecurrentpasses' only at the time t7-and said discharge is interrupted atthe time 18. The following discharge has the same shape as the firstdischarge, the voltage being applied as early as t9 and the currentpassing between H0 and tll.

FIG. 3b shows thepassage of the discharge current 1 for eachcase=illustrated in FIG. 3a and it is apparent that each discharge isperformed under constant current conditions and lasts for apredetermined duration. Thus, all the discharges are equal as concernstheir energy. The curve illustrated in FIG. 3b corresponds also to theinput voltage which is applied to the Schmitt trigger 2. The outputsignal of the latter is similar to the input signal, but it is shaped;in other words, the output signal passes suddenly from a stationarylevel to another stationary level even when the input signal isconstituted by voltage pulses of a variable level.

FIG. 3c illustrates the voltage pulses at the output of thedifferentiating circuit 14 fed by the Schmitt trigger FIG. 3dillustrates the operation of the monostable multivibrator 3 which passesfrom its stable condition I to its non-stable condition II each time itreceives a positive pulse illustrated in FIG.:3c showing theestablishment of the discharge current. Since multivibrator 3 ismonostable, it'returns automatically into its condition I apredetermined time Tp after it has been brought into its non-stablecondition II. The time delay Tp is selected so as to be equal to thedesired duration of the discharge for predetermined machiningconditions. It may be readily modified through a change in the value ofacondenser capacity and/or of a resistance in the multivibrator 3.

FIG. 3e illustrates the pulses. obtained at the output of thedifferentiating circuit IS, the polarity of which is predeterminedduring the rocking of the multivibrator 3 into one of its conditions andof the opposite polarity when it rocks into its other condition. Thesepulses are applied to the monostable multivibrator 4 to drive it intoits non-stable condition each time it receives a pulse corresponding tothe end of a discharge between the electrode and the part to bemachined.

FIG. 3fshows that the multivibrator 4 passes from its stable condition Ito its non-stable condition [I each time the differentiating circuit 15supplies a pulse of a predetermined polarity. Since the multivibrator 4is monostable, it returns into its position I a predetermined time afterit has rocked into its position II. The time delay is selected so as tobe equal to the desired duration of opening Toff of the dischargecircuit after each discharge. The output signal of the multivibrator 4serves for controlling the current breaking and making element 1, so asto make the latter conductive when the multivibrator 4 is in itscondition I and to produce the non-conductive condition of the element 1when the multivibrator 4 is in its condition II. At the time t it willbe assumed that the multivibrator 4 has been brought into its conditionII, so that it rocks into is condition I at the time t1 and closes theelement 1. The voltage of the supply S is then applied between theelectrode E and the part P and at the moment t2 when the dischargecurrent is established, the variations in voltage across the terminalsof R fed to the trigger 2 lead to the formation of a voltage pulse p atthe output -of multivibrator 3 defines the duration Tp of a dischargebetween E and P since, when the multivibrator 3 returns to its conditionI, there is produced through the differential circuit a pulse q whichbrings the multivibrator 4 into its condition II, this producingsimultaneously the opening of the discharge circuit through theelement 1. The duration of opening of element 1 is defined by theduration Toff required for the multivibrator 4 to return into its stablecondition. Thus, discharges are obtained which have identical durationsTp as appears from inspection of FIG. 3b whereas the time Toffseparating the end of one discharge form the moment as which the voltageadapted to produce the next voltage is applied is always the same, asapparent from inspection of FIG. 3a.

FIG. 4 is a circuit diagram of an embodiment corresponding to FIG. 2. Onthe right-hand side of FIG. 4 is shown the supply S, the part to bemachined P, the electrode E and the resistance R, which connectedbetween the electrode E and the current making and breaking element 1,which is constituted by a battery of transistors in parallel, of whichonly two have been illustrated and are designated by the references T1and T2. The collectors of these two transistors are each in series witha resistance R3 or R4 of a reduced value, said resistances R3 and R4being adapted to equilibrate the current in the usual case where thecharacteristics of the transistors are not perfectly identical. Thedischarge circuit includes also a resistance R5 adapted to limit anddefine the amplitude of the pulse formed by the discharge current,resistance R5 being advantageously adjustable.

The voltage appearing across the terminals of the resistance Rl when itis fed by the discharge current, is fed by the leads 5 and 6 to theinput of the Schmitt trigger 2 which includes in the conventional mannertwo transistors T3 and T4. Transistors T3 and T4 are of the npn type inthis case and they are fed by a supply of current 8,. Their emitters areinterconnected and are connected with a terminal of said supply Sthrough a resistance R6. Their collectors are connected with the otherterminal of the supply S, through a resistance R7 and R8. The collectorof the transistor T3 is connected with the base of T4 through aresistance R9 bridged by a condenser C1. The signal tapped off theresistance R] is applied to the base of T3 through a resistance in R Thedifferent resistances of the Schmitttrigger 2 are given values such thatthe transistor T4 operates while T3 is locked as long as the signalapplied to the base of T3 remains lower than a predetermined value,example +5V. If the signal rises above 5V, the transistor T3 be comesconductive and the drop in voltage appearing on its collector produces alocking of the transistor T4. This operative condition remains unalteredas long as the input signal does not drop below a predetermined value,for example 3V. Thus, even in the case where the discharge current issubjected to substantial modifications in an untimely manner, forinstance as a consequence of an erroneous adjustment of the spacingbetween the electrode and the part, there is obtained at the output ofthe Schmitt trigger a clear signal which does not reproduce theundesired modifications.

The collector of the transistor T4 is connected with a network supplyingan electric signal at the output which corresponds to the derivative ofthe input signal. This network is constituted by a condenser C2 and aresistance R the connection between these condenser and resistancesupplying a positive pulse at the moment of the locking of thetransistor T4 and a negative pulse at the moment at which the latterbecomes again conductive. A diode D1 passes only positive pulses to theinput of the monostable multivibrator 3 which includes two transistorsT5 and T6 of the npn type, of which transistors the emitters areconnected with the negative pole of the supply S1. The collectors of T5and T6 are connected through resistances R and R with the positive poleof the supply 51. The collector of T6 is furthermore connected through aresis tance R shunted by a condenser C6 to the base of the transistor T5whereas the collector of T5 is connected with the base of T6 through anRC network constituted by a variable condenser C3 and a resistance R Inthe absence of a signal on its base, the transistor T5 is locked and thetransistor T6 is conductive. When a positive pulse is applied to thebase of T5, the transistor becomes conductive and locks the transistorT6 through a negative pulse transmitted through C3. The transistor T6remains locked as long as the condenser C3 has not been discharged intothe resistance R When the potential at the terminal of C3, which isconnected with the base of T6, is no longer negative and becomesslightly positive as a consequence of the discharge of condenser C3, T6becomes again conductive. The multivibrator 3 is thus in a stablecondition and it can be shifted out of this condition for apredetermined period under the action ofa positive pulse applied to thebase of T5. Multivibrator 3 is therefore of a monostable type and thepotential of the collector of the transistor T5 forms the output signalapplied to a RC circuit of the same type as that connecting the Schmitttrigger with the multivibrator 3.

The R.C. circuit constituted by a condenser C4 and a resistance R16controls the second monostable multivibrator 4 through positive pulsesapplied to the base of the transistor T7. The circuit of themultivibrator 4 is similar to that of the multivibrator 3 and thecollector of T7 is connected with the base of the transistor T8 througha RC circuit including a condenser C5 and a resistance R The collectorof T8 is connected with the base of T7 through a resistance R shunted bya condenser C7. The collectors of the transistors T7 and T8 areconnected with the positive pole of supply S through the resistance Rand R20, respectively.

Multivibrator 4 operates in the same manner as that which has just beendescribed. The transistor T8 is normally conductive and it is broughtinto its nonconductive condition for a predetermined period each time apositive pulse is applied to the base of the transistor T7.Multivibrator 4 controls the circuit making and breaking element throughan amplifier including a transistor T9 of the pnp type which is fed by aseparate supply S2. The potential of the collector of the transistor T7is applied to the base of T9 through a voltage divider includingresistances R21 and R22. The potential of the collector of T9 is appliedto the base of the transistors T and T by a voltage divider R23 and R24.The collector of T9 is connected with the negative terminal of S2through a resistance R25. Through this arrange ment, the transistors Tand T are made conductive and non-conductive is synchronism with thetransistor T8 as readily apparent from the following disclosure.

The collector of the transistor T7 is at a potential which issubstantially equal to that of its emitter when the transistor isconductive, whereas when it is nonconductive, the potential of thiscollector is more positive. The base of the transistor T9 follows thecollector in potential and, according to whether the transistor T7 isconductive or non-conductive, it is negative or positive with referenceto the emitter of the transistor T9, since R22 is connected with thenegative terminal of 82. Since T9 is of the pnp type, it is conductiveand/or non-conductive at the same time as T7.

When T9 is conductive, the potential of its collector is substantiallyequal to that of its emitter, whereas when, it is non-conductive, thepotential is more negative. The potential of the collector of T9 istransmitted through resistance R23 to the bases of the transistors T andT The resistances R23, R24 and R25 are selected in a manner such thatwhen T9 is conductive, the bases of T, and T may be positive withreference to the emitters of said transistors, which renders transistorsT1 and T2 nonconducting, whereas when t9 is nonconductive, the negativevoltage of S2 is applied to the voltage divider constituted by R25, R23and R24, which gives the bases of T and T2 a potential which is slightlymore negative than that of their emitters, whereby transistors T1 and T2become conductive.

In brief, the transistors T and T are conductive or non-conductive atthe same time as the transistor T8 of the multivibrator 4 due to theoutput signal from tran sistor T8 being amplified by the transistor T9control T and T2.

In the embodiment according to FIG. 5, the same basic circuit elementsutilized as in the case just described but the circuit making andbreaking element is constituted by at least one semiconductor controlledswitch such as those known by the Registered Trade Names Trigistor andTranswitch, and referred to in the drawing as TS. The controlled switchis brought into its conductive condition by a short impulse of apredetermined polarity applied to a special gate or control electrodeand into its non-conductive condition by a short pulse of a reversedpolarity applied to the same elec trode.

In the embodiment considered, the discharge current between the part Pand the electrode E is detected by sensing the voltage between theelectrode and the part. The control voltage is applied to apotentiometer 7 having a slider to allow adjustment of the level of theinput signal into the Schmitt trigger 2. In this case, when dischargecommences, the voltage between E and P is reduced suddenly so that thevoltage at the input of the Schmitt trigger 2 decreases simultaneouslyand causes its rocking. The short pulses of either polarity intended forthe control of TS are formed respectively by differentiating circuits l6and 17 fed by the multivibrators 3 and 4. The diodes D3 and D4 allow thepassage only of pulses of a predetermined polarity respectively throughthe resistances R26 and R27.

The short negative pulses supplied by the differentiating circuit 17 atthe moment at which the multivibrator 3 rocks back from its non-stablecondition in which it remains for a duration Ton into its stablecondition, ensures the opening of the discharge circuit through theswitch TS, whereas the short positive pulses supplied by thedifferential circuit 16 at the moment at which the multivibrator 4 rocksback out of its nonstable condition in which it remains for a durationToff into its stable condition bring the switch TS into its con ductivecondition.

The diagram of operation is the same as that illustrated in FIGS. 3a to3f and the elements 2, 3, 4 are identical with those illustrated in FIG.4. However, in

the present case, the passage of the discharge current detected by alowering in the voltage between the electrode E and the part P locks theinput transistor T3 of the Schmitt trigger 2. In this embodiment, theoutput signal of the Schmitt trigger is thus tapped off the collector ofT3 instead of the collector T4 in FIG. 4. Furthermore, the negativepulses applied to TS are obtained through the shunting of a signaltapped off the collector of T6 in FIG. 4. The positive pulses applied toTS are obtained through shunting of a signal tapped off T7 in FIG. 7.

In the third embodiment illustrated in FIG. 6, the main supply ofmachining current is constituted by a rectifying system fed with threephase current under low voltage through the filtering induction coilslimiting the current as shown at LR, LS and LT. The current rectified bythe rectifiers 8 to 13 is fed into the machining area which is connecteddirectly across the terminals of the supply. The starting of thedischarges is obtained through the agency of an auxiliary supply ofvoltage S3 at for instance V which is higher than the voltage of the arcduring the machining the voltage age of supply S3 being applied betweenthe electrode E and the part P when a npn transistor forming a firstcurrent breaking and making element T providing for the starting becomesconductive. The auxiliary discharge current produced by said supply islimited by two resistances R28 and R29. The resistance R29 servesfurthermore for supplying a voltage detecting the com mencement of thedischarge between the electrode E and the part P.

The discharge current is cut off by the setting of the group oftransistors T, to T, in their conductive condition to form a secondcurrent breaking and making power element which produces a completeshunting of the current fed by the rectifiers 8 to 13 into the machininggap extending between the electrode E and the part P, the shunt formingpractically a short circuit in parallel with said gap. During the shortcircuit produced by the transistors T to T the current increases onlyvery slightly by reason of the regulating action of the induction coilsLR, LS and LT.

The signal detecting the starting of each discharge is produced by thevoltage appearing across the terminals of the resistance R29, whichvoltage is applied to the Schmitt trigger 2 as in the precedingembodiment. Schmitt trigger 2 is followed by a differentiating system 14producing short pulses controlling the multivibrator 3. This arrangementis constituted as in the embodiment of FIG. 4 by a condenser, aresistance and a diode (C2, R and D in FIG. 4).

Similarly, the multivibrator 3 is followed by a differentiating circuit15 supplying short pulses controlling the multivibrator 4. Thisarrangement is similar to that formed by C4, R16 and D2 in FIG. 4. Theoutput signal of the multivibrator 4 is amplified by the transistor Tand then applied to the base of the transistor T so as to render itconductive when the multivibrator 4 is in its stable condition andnon-conductive when the multivibrator 4 is in its non-stable conditionfor a duration Toff. The output signal of the multivibrator 3 servesalso after amplification by an amplifier 16 for the control of a groupof transistors T to T This signal controls the non-conductive conditionof transistors T11 to T13 when the multivibrator 3 is in its non-stablecondition for a duration Ton whereby during this period ofnon-stability, the main rectified current from the mains may be fedbetween the electrode E and the part P and is superposed over theauxiliary current supplied by S3.

It should be remarked that the output signal of the amplifier 16 isassociated through two resistances R30 and R31 with a voltage biasingthe bases of the transitors T to T so as to maintain the latter in theirconductive condition during the stable condition of the multivibrator 3,this voltage being supplied by an auxiliary supply S4 of about 10 volts.

A supply S5 also of about volts feeds the Schmitt trigger 2, themultivibrators 3 and 4, the amplifier 16 and the transistor T In thislatter embodiment, it is very important for the opening of the shortcircuit closed by the transistors T to T to be prevented whenever nodischarge is initiated between the electrode E and the part P by thesupply S3, since there may then be obtained an overvoltage across theterminals of the main supply of the machining current capable ofdestroying the group of transistors. This objectionable occurrence isautomatically avoided through the control of T, to T by the outputsignal of the multivibrator 3. Furthermore, the maximum voltageappearing across the emitters and collectors of the transistors T to Tis limited to the discharge voltage which even transiently is lower thanthe voltage of the auxiliary initiating supply S3. The power diode D5prevents the supply S3 from feeding as a short-circuit the groups oftransistors T to T during the time elapsed between the moment at which Tbecomes conductive and the moment at which the discharge is initiated,that is during the delay of an undefined duration referred to at thebeginning of the present specification.

Obviously, numerous modifications may be brought to the differentembodiments described. In particular. the method forming the object ofthe invention may be executed by resorting to means different from thosedescribed and, for instance, the current making and breaking elementsmay be replaced by vacuum tubes, gas-filled thyratrons and the like.Similarly, the multivi brators, although they are advantageous from astructural standpoint, may be replaced by any other electric diagram orarrangement operating as a timer, that is any arrangement adapted tosupply an output signal after a predetermined time has elapsed after thereception of the input signal.

In view of obtaining perfect constancy of energy for each pulse, it ispossible to resort to special multivibrators 3 and 4 in which theduration of the non-stable condition respectively Tp and/or Toff mayvary automatically in response to the electrical machining conditions.In particular, it is possible to make Tp vary in a direction opposed tothe modifications in the voltage of each discharge and to themodifications in the discharge current in order to maintain at aconstant value the energy of the successive discharges with a very highaccuracy. The current may also be detected by means of a transformer ofwhich the primary is connected in series in the electrode and partcircuit, while its secondary supplies pulses of opposite polaritiesrespectively at the beginning of the establishment of the dischargecurrent and at the moment of the cutting off of said current. Similarly,the RC circuits forming differentiating circuits may be replaced withoutany difficulty by transformers producing in a conventional manner apulse for each sudden modification in the current passing through theprimary.

I claim:

1. Apparatus for machining a part by elctric discharge comprising anelectrode disposed adjacent said part so as to define a gaptherebetween, a voltage source, switching means for applying successivevoltage pulses to said gapfcircuit means connecting said voltage sourceand said switching means with said electrode and said part, means forsensing the commencement of electric discharge across said gap, andtiming means connected with said sensing means and said switching meansand responsive to said sensing means to open said switching means to endsaid electric discharge a predetermined time after commencement of saidelectric discharge whereby electric discharge is assured with each ofsaid pulses.

2. The invention as recited in claim 1 wherein said timing meansincludes means for closing said switching means to initiate a succeedingpulse a predetermined time after said switching means is opened.

3. The invention as recited in claim 1 wherein said circuit meansincludes a resistor connected in series with said switching means andsaid voltage source, and sensing means includes means for sensing thevoltage across said resistor.

4. The invention as recited in claim 1 wherein said timing meansincludes a first monostable multivibrator having an input connected withsaid sensing means and an output, end a second monostable multivibratorhaving an input connected with the output of said first monostablemultivibrator and an output connected with said switching means wherebythe time delay associated with said first monostable multivibratordefines the duration of the electric discharge associated with eachpulse and the time delay associated with said second monostablemultivibrator defines the duration between the end of said electricdischarge and the initiation of the next succeeding pulse.

5. The invention as recited in claim 1 wherein said switching meansincludes a semiconductor device having a control electrode connectedwith said timing means.

6. Apparatus for machining a part by electric discharge comprising anelectrode disposed adjacent said part so as to define a gaptherebetween; a source of voltage; switching means for periodicallyapplying said voltage to said gap to define a succession of voltagepulses; means for sensing the commencement of electric discharge acrosssaid gap; control means for said switching means connected with andresponsive to said sensing means and including first timing; means forcontrolling said switching means so that the duration of the electricdischarge associated'with each of said voltage pulses is constant andsecond timing means connected with end responsive to said first timingmeans for controlling said switching means so that the period betweenthe end of the electric discharge associated with each of said voltagepulses and the initiation of the next succeeding voltage pulse isconstant; and means connecting said control means with said switchingmeans.

7. The invention as recited in claim 6 wherein said first timing meansincludes a first monostable multivibra tor having an input connectedwith said sensing means and an output, and said second timing meansincludes a second monostable multivibrator having an input connectedwith the output of said first monostable multivibrator and an outputconnected with said switching means.

8. The invention as recited in claim 6 wherein said sensing meansincludes a resistor connected in series with said switching means andsaid electrode, and means for sensing the voltage across said resistor.

9. the invention as recited in claim 6 wherein said sensing meansincludes voltage sensing means and a Schmitt trigger connected with andresponsive to said voltage sensing means.

10. The invention as recited in claim 6 wherein said switching meansincludes a semiconductor controlled switch having a control electrode,and said means connecting said control means with said switching meansincludes first pulse forming means connected with said first timingmeans and said control electrode to provide pulses to render saidsemiconductor controlled switch nonconductive and second pulse formingmeans connected with said second timing means and said control electrodeto provide pulses to render said semiconductor controlled switchconductive.

11. The invention as recited in claim 6 wherein said switching meansincludes a plurality of transistors, each having a base, a collector andan emitter and means connecting said transistors in parallel at theircollectors and emitters and connecting their bases together, and saidmeans connecting said control means with said switching means includespulse means connected with said second timing means and said bases forproviding pulses to control the conduction of said plurality oftransistors.

12. Apparatus for machining a part by electric discharge comprising anelectrode disposed adjacent said part to define a gap therebetween,circuit means connected with said electrode for supplying pulses ofelectrical energy to cause electric discharge across said gap, means forsensing the commencement of electric discharge across said gap, timingmeans connecting with and responsive to said sensing means forcontrolling the duration of the electric discharge associated with eachof said pulses and the time duration between successive ones of saidpulses, and means connecting said timing means with said circuit meansto control the operation thereof.

13. The invention as recited in claim 12 wherein said circuit meansincludes a first source of electrical energy, first switching means forconnecting. said first source across said gap, a second source ofelectrical energy connected across said gap and second switching meansconnected across said gap for shunting said second source, said timingsource, said timing means includes first means connected with saidsensing means and having a time delay associated therewith equal to theduration of the electric discharge associated with each of said pulsesand second means connected with said first means and having a time delayassociated therewith equal to the time duration between successive onesof said pulses, and said means connecting said timing means with saidcircuit means includes third means connecting said first timing meanswith said second switching means and fourth means connecting said secondtiming means with said first switching means.

14. The invention as recited in claim 13 wherein said sensing meansincludes a resistor, said first means includes a first monostablemultivibrator and said second means includes a second monostablemultivibrator.

15. In an electrical discharge machine for eroding material from aworkpiece by passing successive pulses through an ionizable work gapdefined between an electrode tool and the workpiece, a system forsupplying said pulses comprising the combination of a first switch meansfor selectively coupling a source of ionizing potential across said workgap, a second switch means for selectively coupling a source of materialeroding energy across said work gap, a detection means coupled acrosssaid gap for providing a first output signal when the gap is ionized,and a pulse generating means coupled to said second switch means andresponsive to said first output signal from said detection means foractuating said second switch means to couple saidsource of materialeroding energy across said work gap only when said work gap is ionized.

16. The system of claim 15 wherein said pulse generating means includesa monostable pulse generator which is coupled to said second switchmeans and which is triggered by said first output signal to actuate saidsecond switch means.

17. The system of claim 16 wherein said pulse generating means includesa monostable pulse generator coupled to said second switch means, abistable circuit means coupled to said first switch means and having afirst and second state, first coupling means coupling said detectionmeans to said bistable means for switching said bistable means to saidfirst state and actuating said first switch means in response to saidsecond output signal, second coupling means coupling said monostablepulse generator with said detection means for actuating said secondswitch means in response to said first output signal, and third couplingmeans coupling said monostable pulse generator and said bistable meansfor switching said bistable means to said second state and deactuatingsaid first switch means in response to the actuation of said secondswitch means.

18. The system of claim 17 wherein said source of ionizing potential hasa relatively high voltage, low current characteristic and said source ofmaterial eroding energy has a relatively low voltage, high currentcharacteristic.

19. The system of claim 18 wherein said gap has a varying ionizationvoltage point, said source of ionizing potential provides a voltagewhich is greater than the ionization voltage point of the gap, and saidsource of material eroding energy provides a voltage which is lower thanthe ionization voltage point of the gap.

20. The system of claim 17 further including a gate circuit coupledbetween said monostable pulse generator and said second switch means,and a reference voltage circuit coupled between said electrode tool andsaid gate circuit for supplying a signal to inhibit said gate circuit inresponse to a drop in the voltage across said gap to a point below apredetermined level to thereby deactuate said second switch means in theevent of a gap short circuit and the like.

21. The system of claim 15 wherein said detection means further providesa second output signal when the work gap is deionized, and wherein saidpulse generating means is coupled to said first switch means and isresponsive to said second output signal for actuating said first switchmeans to couple said source of ionizing potential across the work gapwhen the work gap is deionized.

22. The system of claim 21 wherein said pulse generating means includesa bistable circuit means connected in actuating relation with said firstswitch means and having a first state and a second state, said bistablecircuit means being coupled to said detection means and being switchedto said first state in response to said second output signal foractuating said first switch means and to said second state in responseto said first output signal for deactuating said first switch means.

23. The system of claim 21 further including means coupled to at leastone of said electrode tool and workpiece and responsive to a drop in thevoltage across the gap to a point below a predetermined level to providean inhibiting signal and gate means coupled between for deactuating saidsecond switch means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 916,1338 Dated October 28 1975 Inventor (s) Jean Pfau It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 39, "asoribable" should be --ascribable--.

Column 2, line 15, "shown" should be --show.

Column 2, line 41, after "present" insert invention-.

Column 3, line 59, "how" should be --1-+now--.

Column 5, line 10, "form" should be -from-.

Column 5, line 17, following "which" insert is-.

Column 5, line 48, before "example" insert -for-.

Column 8, line 22, delete "age".

Column 10, line 29, "end" should be --and--.

Column 10, line 55, "end" should be and--.

Signzd and Scaled this third Day of February 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflatemsand Trademarks Patent No.

Inventor (s) Jean Pfau Dated October 28, 1975 It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column ascribable.

Column Column --invention-.

Column Column Column Column Column Column Column [SEAL] 1, line 39,

line 15,

line 41,

line 59,

line 10,

line 17,

line 48,

line 22,

10, line 29,

10, line 55,

Arrest:

RUTH C. MASON Arresting Officer "asoribable" should be "shown" should beshow.

after "present" insert "how" should be *fnOW-"u "form" should be -from.

following "which" insert is-.

"end" should be --and-.

Signed and Sealed this third D ay 0f February I 976 C. MARSHALL DANNCommissioner ufParents and Trademarks CERTIFICATE Patent No. 3 916, 138

OF CORRECTION Dated October 28, 1975 Inventor (s) Jean Pfau It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 39,

--ascribable-.

Column line 15,

Column invention--.

line 41,

Column line 59,

Column line 10,

Column line 17,

Column line 48,

Column line 22,

Column 10, line 29,

Column 10', line 55,

[SEAL] Attest:

RUTH C. MASON Arresting Officer "asoribable" should be "shown" should be-show-.

after "present" insert should be and--.

"end should be and--.

Signed and Sealed this third Day of February 1976 C. MARSHALL DANNCommissioner nflarenrs and Trademarks

1. Apparatus for machining a part by elctric discharge comprising anelectrode disposed adjacent said part so as to define a gaptherebetween, a voltage source, switching means for applying successivevoltage pulses to said gap, circuit means connecting said voltage sourceand said switching means with said electrode and said part, means forsensing the commencement of electric discharge across said gap, andtiming means connected with said sensing means and said switching meansand responsive to said sensing means to open said switching means to endsaid electric discharge a predetermined time after commencement of saidelectric discharge whereby electric discharge is assured with each ofsaid pulses.
 2. The invention as recited in claim 1 wherein said timingmeans includes means for closing said switching means to initiate asucceeding pulse a predetermined time after said switching means isopened.
 3. The invention as recited in claim 1 wherein said circuitmeans includes a resistor connected in series with said switching meansand said voltage source, and sensing means includes means for sensingthe voltage across said resistor.
 4. The invention as recited in claim 1wherein said timing means includes a first monostable multivibratorhaving an input connected with said sensing means and an output, end asecond monostable multivibrator having an input connected with theoutput of said first monostable multivibrator and an output connectedwith said switching means whereby the time delay associated with saidfirst monostable multivibrator defines the duration of the electricdischarge associated with each pulse and the time delay associated withsaid second monostable multivibrator defines the duration between theend of said electric discharge and the initiation of the next succeedingpulse.
 5. The invention as recited in claim 1 wherein said switchingmeans includes a semiconductor device having a control electrodeconnected with said timing means.
 6. Apparatus for machining a part byelectric discharge comprising an electrode disposed adjacent said partso as to define a gap therebetween; a source of voltage; switching meansfor periodically applying said voltage to said gap to define asuccession of voltage pulses; means for sensing the commencement ofelectric discharge across said gap; control means for said switchingmeans connected with and responsive to said sensing means and includingfirst timing means for controlling said switching means so that theduration of the electric discharge associated with each of said voltagepulses is constant and second timing means connected with and responsiveto said first timing meAns for controlling said switching means so thatthe period between the end of the electric discharge associated witheach of said voltage pulses and the initiation of the next succeedingvoltage pulse is constant; and means connecting said control means withsaid switching means.
 7. The invention as recited in claim 6 whereinsaid first timing means includes a first monostable multivibrator havingan input connected with said sensing means and an output, and saidsecond timing means includes a second monostable multivibrator having aninput connected with the output of said first monostable multivibratorand an output connected with said switching means.
 8. The invention asrecited in claim 6 wherein said sensing means includes a resistorconnected in series with said switching means and said electrode, andmeans for sensing the voltage across said resistor.
 9. The invention asrecited in claim 6 wherein said sensing means includes voltage sensingmeans and a Schmitt trigger connected with and responsive to saidvoltage sensing means.
 10. The invention as recited in claim 6 whereinsaid switching means includes a semiconductor controlled switch having acontrol electrode, and said means connecting said control means withsaid switching means includes first pulse forming means connected withsaid first timing means and said control electrode to provide pulses torender said semiconductor controlled switch nonconductive and secondpulse forming means connected with said second timing means and saidcontrol electrode to provide pulses to render said semiconductorcontrolled switch conductive.
 11. The invention as recited in claim 6wherein said switching means includes a plurality of transistors, eachhaving a base, a collector and an emitter and means connecting saidtransistors in parallel at their collectors and emitters and connectingtheir bases together, and said means connecting said control means withsaid switching means includes pulse means connected with said secondtiming means and said bases for providing pulses to control theconduction of said plurality of transistors.
 12. Apparatus for machininga part by electric discharge comprising an electrode disposed adjacentsaid part to define a gap therebetween, circuit means connected withsaid electrode for supplying pulses of electrical energy to causeelectric discharge across said gap, means for sensing the commencementof electric discharge across said gap, timing means connecting with andresponsive to said sensing means for controlling the duration of theelectric discharge associated with each of said pulses and the timeduration between successive ones of said pulses, and means connectingsaid timing means with said circuit means to control the operationthereof.
 13. The invention as recited in claim 12 wherein said circuitmeans includes a first source of electrical energy, first switchingmeans for connecting said first source across said gap, a second sourceof electrical energy connected across said gap and second switchingmeans connected across said gap for shunting said second source, saidtiming source, said timing means includes first means connected withsaid sensing means and having a time delay associated therewith equal tothe duration of the electric discharge associated with each of saidpulses and second means connected with said first means and having atime delay associated therewith equal to the time duration betweensuccessive ones of said pulses, and said means connecting said timingmeans with said circuit means includes third means connecting said firsttiming means with said second switching means and fourth meansconnecting said second timing means with said first switching means. 14.The invention as recited in claim 13 wherein said sensing means includesa resistor, said first means includes a first monostable multivibratorand said second means includes a second monostable multivibrator.
 15. Inan electrical discharge machine for eroding material from a workpiece bypassing successive pulses through an ionizable work gap defined betweenan electrode tool and the workpiece, a system for supplying said pulsescomprising the combination of a first switch means for selectivelycoupling a source of ionizing potential across said work gap, a secondswitch means for selectively coupling a source of material erodingenergy across said work gap, a detection means coupled across said gapfor providing a first output signal when the gap is ionized, and a pulsegenerating means coupled to said second switch means and responsive tosaid first output signal from said detection means for actuating saidsecond switch means to couple said source of material eroding energyacross said work gap only when said work gap is ionized.
 16. The systemof claim 15 wherein said pulse generating means includes a monostablepulse generator which is coupled to said second switch means and whichis triggered by said first output signal to actuate said second switchmeans.
 17. The system of claim 16 wherein said pulse generating meansincludes a monostable pulse generator coupled to said second switchmeans, a bistable circuit means coupled to said first switch means andhaving a first and second state, first coupling means coupling saiddetection means to said bistable means for switching said bistable meansto said first state and actuating said first switch means in response tosaid second output signal, second coupling means coupling saidmonostable pulse generator with said detection means for actuating saidsecond switch means in response to said first output signal, and thirdcoupling means coupling said monostable pulse generator and saidbistable means for switching said bistable means to said second stateand deactuating said first switch means in response to the actuation ofsaid second switch means.
 18. The system of claim 17 wherein said sourceof ionizing potential has a relatively high voltage, low currentcharacteristic and said source of material eroding energy has arelatively low voltage, high current characteristic.
 19. The system ofclaim 18 wherein said gap has a varying ionization voltage point, saidsource of ionizing potential provides a voltage which is greater thanthe ionization voltage point of the gap, and said source of materialeroding energy provides a voltage which is lower than the ionizationvoltage point of the gap.
 20. The system of claim 17 further including agate circuit coupled between said monostable pulse generator and saidsecond switch means, and a reference voltage circuit coupled betweensaid electrode tool and said gate circuit for supplying a signal toinhibit said gate circuit in response to a drop in the voltage acrosssaid gap to a point below a predetermined level to thereby deactuatesaid second switch means in the event of a gap short circuit and thelike.
 21. The system of claim 15 wherein said detection means furtherprovides a second output signal when the work gap is deionized, andwherein said pulse generating means is coupled to said first switchmeans and is responsive to said second output signal for actuating saidfirst switch means to couple said source of ionizing potential acrossthe work gap when the work gap is deionized.
 22. The system of claim 21wherein said pulse generating means includes a bistable circuit meansconnected in actuating relation with said first switch means and havinga first state and a second state, said bistable circuit means beingcoupled to said detection means and being switched to said first statein response to said second output signal for actuating said first switchmeans and to said second state in response to said first output signalfor deactuating said first switch means.
 23. The system of claim 21further including means coupled to at least one of said electrode tooland workpiece and responsive to a drop in the voltage across the gap toa point below a predetermined level to provide an inhibiting signal andgate mEans coupled between said pulse generating means and said secondswitch means and responsive to said inhibiting signal for deactuatingsaid second switch means.
 24. The system of claim 15 further includingmeans coupled to at least one of said electrode tool and workpiece andresponsive to a drop in the voltage across the gap to a point below apredetermined level to provide an inhibiting signal, and gate meanscoupled between said pulse generating means and said means second switchmeans and responsive to said inhibiting signal for deactuating saidsecond switch means.